Video: Removing ghosts, correcting backlighting

We're still working away inside the Merge to HDR Pro utility. We've gone ahead and modified all of the numerical settings here inside this Local Adaptation area. In this exercise, we need to address two very specific luminance problems. Over here in the upper right-hand corner, we have these strange, colorful artifacts around these highlights on the wood, and over here on the left-hand side, in this doorway, we have so much of backlighting that it's just blowing away the slats of wood, and I need to figure out some way to calm that down.

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Removing ghosts, correcting backlighting

We're still working away inside the Merge to HDR Pro utility.We've gone ahead and modified all of the numerical settings here inside thisLocal Adaptation area.In this exercise, we need to address two very specific luminance problems.Over here in the upper right-hand corner, we have these strange, colorfulartifacts around these highlights on the wood, and over here on the left-handside, in this doorway, we have so much of backlighting that it's just blowing awaythe slats of wood, and I need to figure out some way to calm that down.

These are two very different problems with two very different solutions, as you might imagine.Let's start with what is known as ghosting right here.These little weird edges with these red blobs and cyan blobs and yellow blobs, and so on.That's a function of something moving in the scene.Now, sometimes it's obvious what's moving.Let's say you shoot a bunch of exposures in a park, and there is some jogger on the trail.Well, the jogger is couldn't be surrounded by these weird colorful artifacts, andvery possibly the jogger himself or herself will appear gray, just likeunnaturally, completely gray without any detail inside the person at all.

And what you've got, by the way, in this case, this moving item that shows upincorrectly throughout the composition, it's known as a ghost.Now, in our case, you might wonder how in the world we have ghosting, becausethat's what this is.After all, this scene has no motion in it whatsoever.I was unaware of a single organism moving around inside of this barn.I'm sure there were some microorganisms, but there was neither a mouse, nor adeer, nor anything like that and yet, something's moving right here, and it's not a ghost.

What it is the sunlight, because the Earth continues to spin on its axis,especially when you're doing a 20-second exposure, I mean you can reallytrack that kind of stuff,why then the light does appear to move.And over the course of many shots, it probably took me three or four minutes toshoot all of these exposures,why then I had a lot of movement to track.So, what you do is you go up to this check box right there, Remove ghosts, andyou say, Photoshop, get rid of ghost please. And when you select that checkbox, it does take a few moments to calculate because Photoshop is trying todecide, all right, if that's a ghost, which one of the images has the rightversion of that ghost?If the jogger is at a different position across each and every one of theseimages, which one of the images is right?Well, it goes ahead and tells you which image is right with this greenborder right there.

I don't know why they chose green, but they did;maybe green for go.That is the image now, upon which this moving highlight is based.If you don't want the first image to be the representative image, then you justgo ahead and click on a different on.For example, I'm going to click on this fourth image in, which I believe is thefive-second exposure, in order to switch out the ghost like so. And you can seewe get very different highlights this time, and they looked much, much better.All right, I'll zoom out, and you can sort of scan around for other ghostsinside the image if you want to.

That was the worst of it, although there is a little bit of ghosting surroundingthis lava-like highlight right there in the middle of things.Anyway, now we need to direct our attention toward the slats, and the way thatwe're going to downplay this light - just a little bit.we're not going to get rid of it or anything, but we can send it into remission a bit -is not with any of these numerical settings.You can work with these guys till you're blue in the face, and you're not likelyto get very good results out of them, at least where this problem is concerned.Instead, we want to take advantage of the curve.We're working in a crazy space.

You have to bear that in mind, when we're working in the 16-bit per channel mode,that means we are working in a very big room.Imagine that you're sitting in the audience in a large room, essentially.When you're working in an 8-bit per channel space, there's just 256 chairs in that room.When you're working in a 16-bit per channel space, it's not as if you're justdoubling the number of chairs;you're taking those 256 chairs, and you're multiplying them by 256.So you've got just thousands and thousands of chairs in a room.

The room grew to epic proportions.Now then, when you go to 32-bit per channel, not only do you have 256 times 256chairs, but then you've got 256 times 256 rooms, full of those 256 times 256chairs, and where this analogy is concerned, you can walk through the walls,because you really can't move between the rooms as much as you want. But wherethe viewer is concerned, somebody looking at your photograph here, you can onlysee the contents of a single room at a time.

So there are chairs upon chairs inside rooms upon rooms that you cannot even see.Now, I know that it doesn't make a lick of sense, but it might help youunderstand why the curve does not make a lick of sense either.So, when you're working in a true high bit depth space like this, afloating-point calculation space, then black and white have different meanings.For example, if I just take this white point right here, and I move it allthe way down to the bottom of the graph, basically saying everything should turn black,everything does not turn black, because we just basically didn't manage tocapture everything inside the image.

So, I'm going to have to beg your indulgence while I show you what you do inthe case of this image.Basically, you want to go ahead and surround this histogram, which is totallyover here on the shadow side of the graph.That doesn't mean anything, because bear in mind that this entire length ofgraph, that might represent everything that's available to us, every luminancelevel that's out there, and we can only see this tiny little bit of it at anygiven moment in time.So you don't even have to worry about the lack of highlights over here onthe right-hand side.

We can't see them anyway.What I'm going to do is basically surround the existing histogram, almost likeI'm lassoing it with a curve.So, I'll click in the middle here, and I'll drag that guy up like so, andthen I'll take this endpoint that would normally represent white, and I'lldrag it down to the pits of blackness in order to curtail the Highlights inside the image.Notice as a result, I went ahead and drew in, essentially, a slat that hadbeen formerly missing.

So, when this point was all the way up there, we had this brightness, so muchbacklighting that the middle slat had gone away.However, if I drag this point down to about here, and notice I am in this thirdsort of quadrant right there, two red tick marks in,then I bring this bright slat back into the realm of visibility.Now, I'm going to grab this midpoint here, and the values I came up with were anInput level of 30% and then an Output level of 33%,although an Output level of 30% I believe works just fine too. But here is thefinal result, with this strange curve employed in order to rein back in thehighlights over here on the left-hand side and of course the Remove ghost optionthat very successfully got rid of the color artifacting around the highlights inthe upper right-hand corner of the image.

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Learn by watching, listening, and doing! Exercise files are the same files the author uses in the course, so you can download them and follow along. Exercise files are available with all Premium memberships.
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